1. Field of the Invention
The present invention relates to an electrolytic solution used in an electrochemical cell such as a secondary battery and an electric double layer capacitor, and an electrochemical cell such as a secondary battery and an electric double layer capacitor that uses such an electrolytic solution; more specifically the present invention relates to an electrolytic solution to prevent the oxidation and degradation of electrode active materials, and to improve the cycle properties in a cell that includes a proton-conducting polymer as an active material.
2. Related Art
An electrochemical cell, such as a secondary battery and an electric double layer capacitor, that uses a proton-conducting polymer as an electrode active material (hereafter referred to as “cell”) has been proposed and put to practical use. Such a cell has a constitution, for example as shown in FIG. 1, wherein a positive electrode 2 containing a proton-conducting polymer as the active material on a positive electrode collector 1, a negative electrode 4 on a negative electrode collector 3, and these are laminated via a separator 5; and only protons involve as electric charge carriers. An aqueous or non-aqueous solution containing a proton source is filled as an electrolytic solution. Reference numeral 6 denotes a gasket.
The electrodes are formed on collectors, such as a conductive rubber sheets, by slurry prepared from the powder of a doped or non-doped material polymer (proton-conducting polymer), a conducting additive, and a binder, which is put in a mold of a desired size and compressed with a hot press to have desired electrode density and film thickness. Thus formed positive electrode and negative electrode are arranged so as to face each other through a separator to constitute a cell.
Proton-conducting polymers used as electrode active materials include π-conjugated polymers, such as polyaniline, polythiophene, polypyrrole, polyacetylene, poly-p-phenylene, polyphenylenevinylene, polyperinaphthalene, polyfuran, polyfurfuran, polythienylene, polypyridinediyl, polyisothianaphthene, polyquinoxaline, polypyridine, polypyrimidine, polyindole, indole trimer, polyaminoanthraquinone, and derivatives thereof; and polymers containing hydroxyl groups (formed from quinone oxygen due to conjugation), such as polyanthraquinone and polybenzoquinone. Redox pairs are formed by doping an appropriate dopant to these polymers, and conductivity appears. By adequately adjusting oxidation-reduction potential difference, these polymers are selected and used as active materials for positive and negative electrodes.
As the electrolytic solution, an aqueous electrolytic solution comprising an aqueous solution of an acid, and a non-aqueous electrolytic solution based on an organic solvent have been known, and in proton-conducting polymers, the former aqueous electrolytic solution is exclusively used because the cell of a particularly high capacity can be provided. The acids include inorganic acids, such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, tetrafluoroboric acid, and hexafluosilicic acid; and organic acids, such as saturated monocarboxylic acids, aliphatic carboxylic acids, oxycarboxylic acids, p-toluene sulfonic acid, polyvinyl sulfonic acid, and lauric acid.
In a cell using such proton-conducting polymers as active materials, the cycle life is shortened by the rise of internal resistance, and this tendency becomes significant with the elevation of temperature. There is a problem of the degradation of long-term stability in a high-temperature atmosphere.
The causes are that the progress of proton adsorption-desorption reaction, which is the charge-discharge mechanism of electrode active materials, is deteriorated, the degrading atmosphere increases, the excessive oxidation of the materials is accelerated especially in high temperatures, and therefore, the progress of degradation increases. It is also the cause that the heat stability of the electrolytic solution is insufficient.
Particularly, the active materials for electrodes are easily degraded under oxidizing conditions. This is considered to be caused by degradation of the proton (H+) adsorption-desorption reaction of active material compounds over time in the charge-discharge mechanism as described below. The contributing factors include the progress of degradation of active materials in an excessive H+ concentration atmosphere compared with an optimal H+ concentration (different between active materials and depending on the number of reactive electrons), whereby doping-dedoping reactivity between the active material and the electrolyte lowers, and reaction cannot proceed smoothly, and charge-discharge ability lowers (called “excessive oxidation degradation”).
Here, an indole-based polymer (indole trimer) used as the active material for a positive electrode, and a quinoxaline-based polymer used as the active material for a negative electrode will be described.
The charge-discharge mechanism of the materials for positive and negative electrodes is shown in the following scheme: where R represents an optional substituent group, and X represents an anion.
Since this phenomenon is especially significant in a high acid concentration atmosphere (low pH), the degradation of the cycle properties is accelerated. Also, since the conductivity of the electrolyte increases, and the reaction with the active material is activated in a high temperature atmosphere, the progress of degradation by oxidation may be enhanced if the concentration of the electrolyte is excessive.
FIG. 2 is a graph showing change in cycle properties by the concentration of the electrolyte (sulfuric acid). As the graph shows, it can be seen that with increase in the concentration of the electrolyte, the capacity ratio lowers in larger number of cycles, and the cycle properties are degraded.
In an atmosphere of low-concentration of the electrolyte, although the cycle properties are satisfactory, the appearance capacities tend to lower. FIG. 3 is a graph showing change in the appearance capacities by the concentration of the electrolyte (sulfuric acid). As the graph shows, it can be seen that with decrease in the concentration of the electrolyte, the appearance capacities lower.
Therefore, the establishment of the optimal composition of the electrolyte (H+ and X−) is required, and this is the problem to improve the cycle properties.